The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus and, more particularly, to an image forming apparatus of the type having an intermediate image transfer element implemented as, for example, a belt for sequentially effecting primary and secondary image transfer steps.
Intermediate image transfer elements for image forming apparatuses of the type described may generally classified into two kinds, i.e., one made of a dielectric material either entirely or only at the surface thereof where toner is to deposit, and the other made of a material having a medium resistance. For the element having a medium resistance, specific surface resistances, materials and resistance control agents are taught in, for example, Japanese Patent Laid-Open Publication Nos. 63-311263, 56-164368, and 64-74571. The conventional elements are implemented as a seamless belt or a drum having a single layer.
The conventional intermediate image transfer element having a medium resistance has a problem that the inside resistance thereof is scattered by about one figure. This, coupled with the fact that the resistance of the element changes due to aging, lowers the quality of images. Another problem is that the element is apt to invite defective images images due to, for example, toner dust or transfer dust, compared to the element made of a dielectric material. Assume that the medium resistance of the element is implemented by dispersing carbon, metal oxide or similar resistance control agent, or filler, in base resin (mainly polycarbonate, polyvinylidene fluoride, ETFE (ethylene tetrafluoroethylene), polyimide or the like). Then, since the filler is dispersed in the base resin in a great amount, it deteriorates the surface of the belt and thereby brings about toner filming, change in the chargeability of toner, and degradation of images.
Generally, the irregularity in the inside resistance of such an image transfer element is attributable to a production line. The one-figure of irregularity is substantially considered to be the limit of state-of-the-art technologies. Regrading the uniformity of an image, the resistance of the element should preferably be uniform in order to avoid an irregular image attributable to irregular image transfer. Particularly, when the element is provided with a relatively high surface resistance of 10.sup.9 .OMEGA./cm.sup.2 or above, the optimal primary transfer bias range for any belt resistance decreases. Hence, it is necessary to control the irregularity in resistance more strictly in order to insure uniform images. Stated another way, uniform images are not attainable when the irregularity amounts to about one figure.
The change in the resistance of the intermediate image transfer element due to aging depends on the material of the element and a resistance control agent. For example, when the major component of the element is an elastomer, the chain structure of an inorganic resistance control agent or similar filler dispersed in the elastomer breaks up due to aging, so that the resistance tends to increase. When the dispersibility of the filler in the material of the element is short, the filler is apt to cohere due to aging and due to a transfer electric field or similar electrical hazard, causing the resistance to decrease. The change in the resistance of the element due to aging has the following side effects. To begin with, the optimal bias for the primary transfer is deviated to lower image quality. Specifically, a change in resistance results in a change in optimal bias and thereby displaces the optimal bias range after the change in resistance from the initially set value. Another side effect is that the uniformity of an image is lowered due to the irregular change in the resistance due to aging. A further side effect is that blurred images and other defective images are produced (under a low resistance condition).
The intermediate image transfer element of medium resistance is inferior to the element made of a dielectric material in respect of transfer dust. This is because a driving force for toner transfer is implemented only by an electric field in the case of the dielectric element, but it is implemented by both of a transfer current and an electric field in the case of the medium resistance element. Therefore, while the dielectric element may be advantageous over the medium resistance element, transfer dust of a degree close to one available with the dielectric element is desired.